Welcome back to CS 124! You are not alone.
This lesson is a lot of fun. We’ll spend some time on our first discussion of Kotlin internals. Specifically, we’ll explain more about what happens when your code is run. Then we’ll discuss immutability in Kotlin.
Let’s warm up with another graded debugging challenge!
Our focus in this class is on teaching you to think computationally and express those thoughts coherently in the Kotlin programming language. Once you do that, you’ll be able to learn other programming languages easily and accomplish other computational tasks.
So we don’t spend a lot of time on exactly what is going on under the hood when Kotlin executes the code that you submit. On some level, it doesn’t matter. As long as the core language constructs have a stable meaning, the internals of how things work don’t matter. When I write:
I expect to see a message displayed 8 times. Kotlin could and might completely change exactly how that is accomplished, but as long as it is accomplished, I’m not going to notice. This is a powerful idea in computer science, even if at times the methods of implementation are somewhat less than practical.
But! In this case having a bit of an understanding of what is happening is actually useful. And, what is actually happening is quite interesting! So let’s peek under the hood…
When you run a Kotlin program there are actually two distinct steps that are taking place:
Let’s talk a bit about when compilation happens, what is produced, and the kind of errors that the compiler can produce during this step.
Now let’s return to the same environment and discuss the execution step and the kind of errors that can occur at that point.
One of the reasons understanding these steps is so important is that they have a relationship to the difference between development and production:
Compiler errors generally only happen in development. That means that only developers see them! In contrast, runtime errors can happen in production! That means they might affect actual users.
Software developers acquire a high degree of patience with broken and crashy software. Users do not. That means that, if you can reduce runtime errors and catch them during development, you will produce better software.
Kotlin represents a new generation of compilers that harnesses the power of modern computers to help catch errors in your code before they affect users. So while Java allows you to do this—this code generates a runtime error:
Kotlin does not—this code generates a compiler error:
Given a nullable
Strings, write a method
toSet that converts it to a
contains all the same
Strings in the original list.
So given a
List containing "test", "me", and "test", you would return a
Set containing "test" and "me".
require that the passed
List is not
Before we conclude, we’ll introduce an important idea in Kotlin: immutability.
We’ve discussed previously how to declare and initialize variables in Kotlin:
We’ve also pointed out that, due to Kotlin’s powerful type inference, even the Kotlin tracks and enforces variable types, we usually don’t need to explicitly provide types.
In the example above, Kotlin will figure out what type
i is from the type of the initial assignment.
8 is an
Int literal, so
i will be inferred to have type
We know this.
valand Immutable Variables
valand Immutable Variables
But what is this
var keyword that we are using above?
It seems somewhat unnecessary at this point.
Using it does help us distinguish between variable declaration and reassignment:
var actually hints at a deeper and more fundamental feature of Kotlin: immutability.
To see how this works, let’s examine
In addition to
val to declare an immutable variable, Kotlin provides a number of other features to support immutability in our code.
Consider the following snippet:
isHot is entirely dependent on the value of
temperature, which in the code above is a
val and so cannot change.
But, because we need to set the value of
isHot inside an
if statement, we have to declare it as a
var so that we can change it in the
Happily Kotlin provides a very elegant way around this problem:
Previously we’ve seen
if expressions are very similar, except that they can be used on the right side of an assignment!
Let’s explore this idea together:
if expressions turn out to be incredibly useful and powerful, and we’ll use them whenever we need to initialize a variable based on a condition.
Not only are they clear and expressive, but they frequently allow us to mark the initialized variable as
val if it does not need to change after the assignment.
Kotlin’s use of
val promote immutability to a first-class citizen in the language.
(Simliar behavior is possible in Java, which Kotlin interoperates with, through the
But it is less central to the design of the language.)
Each time you create a variable you must decide if it can be reassigned (
var) or not (
You may think that this is a bit strange. After all, it’s a variable! Shouldn’t its value be able to change? Sometimes change is necessary. But often it turns out not to be. And, by reducing the number of variables that can change in our programs, we end up writing code that is easier to understand.
Put it this way.
When I create and initialize a
val, I know immediately what its value is and always will be.
In contrast, when I create and initialize a
var, I have to keep an eye out for any places that its value could change unexpectedly in the code that follows.
That makes my code harder to understand.
Immutability is an important software design principle.
So while it may not seem completely clear why this is so important now, starting now we’ll try and take advantages of
val and Kotlin’s other immutability features to reduce the amount of mutability in our code.
String containing words separated by the " " (space) character, write a method
true if any word appears twice in the
Also, you should ignore case: so "Chuchu" and "chuchu" are considered the same word.
require that the passed
String is not
So, for example, given the
String "Wow that is amazing", you would return
but given the
String "Chuchu chuchu xyz" you would return
Our intention is for you to solve this problem using a
Set, but there are also solutions that use a
You should not use a nested loop to solve this problem.
You may want to use the
lowercase method of Kotlin
Next let’s discuss how to approach our next practice homework problem.
This problem is a bit trickier, since we need to determine when to properly insert entries into our
Map, and do some
So let’s discuss how to get started.
Please don’t attempt this practice problem until you are done with yesterday’s homework problem!
Write a method called
parseScript that accepts a single
String and returns a
String contains a script consisting of lines separated by newlines, each with the following format:
For example, here's a simple script:
Geoffrey: What do you think of this homework problem? Ahmed: it's a bit sus Geoffrey: I bet they'll be able to figure it out! Maaheen: We'll be here to help if they need it.
parseScript parses the script and returns a map mapping each character's name to their lines in order.
So, for the script above, the map would contain three keys: "Geoffrey", "Ahmed", and "Maaheen".
List<String> for the key "Geoffrey" would contain the
Strings "What do you think of this homework
problem!" and "I bet they'll be able to figure it out!"
List<String> for the key "Amhed" would contain the
String "it's a bit sus".
A few hints for approaching this problem.
You'll want to use
.split to parse the passed
String into individual lines.
All lines will have the format shown above.
You'll also need to use
.split to split each line into the name and their line of dialog.
You can assume that the character ":" only appears to delimit the name of the rest of the line.
The first time you encounter a character, there will not be an entry in your map for them. So you should check for this, and create the list when appropriate.
There may be extra whitespace around the name or the line of dialogue, so use
You should not need to use any
import statements to solve this problem.
mutableListOf() to create mutable maps and lists when needed.
Please continue to work on yesterday’s more challenging problem. Good luck! Jump on the tutoring site or find us on the forum when you need some assistance. And keep in mind that this is just one problem…
Need more practice? Head over to the practice page.